1. Field of the Invention
The present invention relates to an axial fan, and more particularly, to an axial fan that is used to cool an electronic device and the like.
2. Description of the Related Art
For example, in electronic devices such as a personal computer, a copier and the like, a plurality of electronic parts is housed in a relatively small casing. Therefore, heat that is generated from the electronic parts is confined in the casing, so that the electronic parts can be thermally destroyed, which causes considerable problems. Accordingly, vent holes are formed in a wall surface or ceiling surface of the casing of the electronic device, so that the heat in the casing is radiated through the vent holes. In the meantime, an axial fan is used as a cooling means of the electronic device. The axial fan for cooling the electronic device is required to reduce noise as much as possible and to improve an air volume (flow rate) performance. In order to improve the air volume performance, a shape of a blade or housing structure has been optimized.
For example, JP-A-H8-303391 describes an axial fan which reduces the noise by optimizing a shape of the blade.
FIG. 13 is a front view showing an axial fan described in JP-A-H8-303391 and FIG. 14 is a sectional view taken along a line B-B of FIG. 13. Blades 1 are radially provided on an outer peripheral surface of a cylindrical hub 2 of an impeller, so that the impeller configures so-called forward-swept blades. Specifically, the impeller is configured such that a mounting angle θ of a blade cross section becomes the maximum at near 70% of a diameter of the impeller. According to this configuration, a boundary layer on a negative pressure surface is blown off so as to reduce a vortex, so that noise can be reduced.
However, recently, as the electronic parts become densified and highly efficient, the amount of heat generated from the electronic parts also increases. Accordingly, an axial fan that is used to cool the electronic device having the electronic parts housed therein is required not only to reduce the noise but also to have a large air volume and a high static pressure and to improve the flow rate characteristic.